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4 Answers
4

At the risk of being way too general in my answer, I will say that you should run an index maintenance process regularly. However, your index maintenance process should only rebuild/reorganize the indexes that specifically require it.

This presents the question: when does an index require to be rebuilt or reorganized? Rolando touched on this nicely. Again, I risk being extremely broad. An index requires maintenance when the fragmentation level adversely affects performance. This level of fragmentation could vary based on the size and composition of the index.

Speaking for SQL Server, I tend to choose a index size and index fragmentation level at which point I begin performing index maintenance. If an index contains less than 100 pages, I will perform no maintenance. If an index is between 10% and 30% fragmented, I will REORGANIZE the index and UPDATE the statistics. If an index is over 30% fragmented, I will REBUILD the index (no UPDATE STATISTICS necessary, as this is taken care of by the REBUILD).

This answer is really just a long way to say: Yes, you should do routine index maintenance, but only on the indexes that need it.

This is more or less what we currently do at our company. Thank you for the concrete recommendations.
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Nick ChammasAug 4 '11 at 1:28

2

Be careful if you don't update stats on tables where you rebuild the index. Rebuilds only update the index driven stats. Stats aren't updated on stats which are build on columns which aren't indexed.
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mrdenny♦Aug 4 '11 at 18:55

Great point, and one that I am aware of, but neglected to mention. I typically handle updating column statistics in a separate process.
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Matt MAug 5 '11 at 0:52

Don't know how to say it better -- the index wants to be big and fat with extra space. It is on a column you update -- moving the index entry from place to place in the index. One day the row has a code of "A", the next day the code is "G", then "Z" then "H" and so on. So the index entry for the row moves from place to place in the index. As it does so, it needs space -- will, if the space isn't there, we split the block into two -- and make space. Now the index is getting fat. Over time the index is 2-3x the size it was when you started and is "half or more empty" But that is OK since you move rows around. Now when we move the rows around, we no longer have to split blocks to make room -- the room is already available.

Then you come along and rebuild or drop and recreate the index (which have the same effects -- just the rebuild is "safer" -- doesn't stand a chance of losing the index and can be faster as the index can be rebuilt by scanning the existing index instead of scanning the table and sorting and building a fresh index). Now, all of that nice space is gone. We start the process of splitting the blocks all over again -- getting us right back to where we started.

You saved no space.

The index is right back the way it was.

You would just be wasting your time to rebuild it again causing this vicious cycle to repeat itself.

The logic here is sound, but it is biased against a read-heavy load profile.

A "fat" index (i.e. one with lots of gaps) does indeed keep a good amount of room for new and moved rows, thus reducing page splits and keeping your writes speedy. However, when you read from that fat index you'll have to read more pages to get the same data because you're now sifting through more empty space. This slows your reads down.

So, in read-heavy databases you want to regularly rebuild or reorganize your indexes. (How often and under what conditions? Matt M already has a concrete answer to this question.) In databases that experience roughly equivalent read and write activity, or in databases that are write-heavy, you are likely harming your database's performance by rebuilding indexes regularly.

+1 - excellent answer. In your question you mention SQL Server and I do wonder if there is something fundamentally different about that platform that makes it different to Oracle, as this question is one that tends to polarise the two sets of DBAs
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Jack Douglas♦Aug 4 '11 at 15:56

I would love to know that as well. Is it just cultural? I wonder if I should remove the reference to SQL Server in my original question to attract answers for the other platforms.
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Nick ChammasAug 4 '11 at 19:30

Most people rebuild them on a regular basis so that they never get to fragmented. When you need to rebuild them is based on how quickly they get fragmented. Some indexes will need to be rebuilt often, others basically never. Check out the script the SQLFool put together that handles a lot of figuring this stuff out for you.